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String Theory for Dummies

  1. Sep 13, 2008 #1
    I am kind of a newbie to string theory. I just read Brain Greene’s “The Elegant Universe”. I thoroughly enjoyed it -- almost a "String Theory for Dummies". Greene certainly has a way of dumbing down theoretical physics (substituting analogy for mathematics) so that even someone like myself can get the basic idea. That said, his book left several questions unanswered. I am hoping that some experts in this forum might be able to help.

    String theory, as I understand it, says that all of the particles in the Standard Model are really infinitesimally small vibrating strings and that the physical properties of the fundamental particle (such as mass and charge) are all properties of the frequency of vibration. In some ways, this in intuitive. Einstein’s equation e=mc^2 says, among other things, that the mass of a particle is proportional to its energy. So if a string vibrates at a higher frequency, it will “appear” more massive.

    Here’s where I get confused. I can understand why mass, and perhaps charge, are functions of the string’s vibration frequency. But I don’t understand the size connection. The diameter of a proton is 1.65e−15m. Its constituent string is substantially smaller, probably on the order of the Plank length (i.e. 1.6e-36m). If a proton is really a Plank length string, what gives the proton its “size”?

    I know I probably did not ask a coherent question so maybe a visualization would be better. If a proton were magnified to the size of the earth, its constituent string, enlarged by the same factor, would still be ten times smaller than the original size of the proton (if, in fact, strings are Plank length in size.) That’s mind boggling to me. So my basic question is this: What are we seeing when we “see” a proton? Or what is it that we are measuring when we say that a proton is 1.65e-15m in diameter?

    Another thing: How do quarks figure into any discussion of string theory? If a proton is said to be a string but quantum physics says a proton is made up of three quarks, then does this mean that a proton is actually made up of three strings?

    And a final question: Are strings uniform in nature? I would assume that there are no such things as electron strings and proton strings. Else, the physical properties of a proton or electron could, on some level, be a function of the string infrastructure and not just the vibrating frequency which seems to run counter to what string theory suggests. Say you could change the vibration frequency of a string (and I realize that this is an impossibility as far as we know but suppose you could). Could you then change a proton into an electron or vice versa?
  2. jcsd
  3. Sep 13, 2008 #2


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    I'm sure you will get much better answers from someone else, but like you suspect since a proton is made up of three quarks, the current idea as far I know, is that a proton is kind of made up three strings where the quarks are somehow associated to the open ends. So I think the current idea is that the proton would be like an string with three ends, "or three strings that are connected", and I suppose the quark/colour confinement is "explained" by "stretching of the strings" opposes quark separation.

  4. Sep 14, 2008 #3


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    I'm no expert in string theory at all. But as far as I know:

    When you talk about the size of a proton or of an atom (or of any composite particle) you
    usually talk about the radius that surrounds all the particle that build up the proton or atom etc.. So it is more a measure of how far spread out the fundamental particles are.

    Take the example of a proton which consists of three quarks. Now you can shoot a couple
    of electrons inside the proton and due to the different electromagnetic interactions a couple
    of different directions of deflection (so to speak) form. From this we can infer that protons
    are in fact composite particles and we can determine that there is a lot of empty space between them.

    In the standard model quarks (and other fundamental particles) are regarded as points in the
    mathematical sense (thus having no width or height). In string theory it is then assumed that
    these fundamental strings are not points but one-dimensional objects.
    However, because their length is so small they are at last when it comes to the size of an atom essentially point like.

    To your last question: You can convert particles into each other or into photons(energy).
    Of course, conservation of energy has to be taken into consideration.

    Hope that helps
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